Turbines are known in the art for producing energy. A typical turbine includes alternating stages of stationary vanes or nozzles and rotating blades or buckets. The rotating buckets are attached to a rotor. A working fluid, such as steam or combustion gases, flows along a hot gas path across the stationary vanes and rotating buckets. The stationary vanes direct the working fluid onto the rotating buckets, causing the rotating buckets, and thus the rotor, to rotate to produce work. For example, the rotor may be connected to a generator so that rotation of the rotor produces electrical energy. Increasing the temperature of the working fluid generally increases the thermodynamic efficiency of the turbine; however, the increased temperature of the working fluid may also result in excessive heating of the turbine buckets and other components along the hot gas path. Therefore, various systems and methods are known in the art for providing cooling to the turbine buckets to prevent damage and/or increase the operating life of the turbine buckets.
The rotating buckets generally comprise an airfoil that extends from a platform into the hot gas path. The rotating buckets further include a shank radially inward of the platform, and the shank often includes a shank cavity. One system and method known in the art for cooling turbine buckets flows a cooling medium into the shank cavity to cool the shank. The cooling medium may comprise any fluid capable of removing heat from the shank cavity, such as diverted air from a compressor. The pressure of the cooling medium flowing into the shank cavity is generally maintained greater than the pressure of the working fluid flowing over the airfoil in the hot gas path. In this manner, the cooling medium prevents the working fluid from bypassing the airfoil and leaking or being ingested into the shank cavity.
The pressure difference between the cooling medium and the working fluid may result in a first portion of the cooling medium leaking out of the shank cavity and into the hot gas path. The first portion of the cooling medium that leaks into the hot gas path then passes through the alternating stages of stationary vanes and rotating buckets to produce work. However, the pressure difference between the cooling medium and the working fluid may also cause a second portion of the cooling medium to flow downstream in the shank cavity and leak out of the shank cavity into a downstream component, such as a wheel space purge cavity downstream of the shank. The second portion of the cooling medium that leaks out of the shank cavity into the downstream component produces no work in the turbine and therefore does not contribute to the thermodynamic efficiency of the turbine.
An aft seal pin may be installed in the aft portion of the shank cavity to reduce the amount of cooling medium that leaks out of the shank cavity into the downstream component. However, the pressure of the cooling medium may still result in unwanted leakage of the cooling medium past the aft seal pin and out of the shank cavity. Therefore, an improved system and method for cooling turbine buckets that reduces the amount of unwanted leakage of the cooling medium out of the shank cavity would be useful.